Microelectronic devices, such as semiconductor devices and field emission displays, are generally fabricated on and/or in microelectronic workpieces using several different types of machines (“tools”). Many such processing machines have a single processing station that performs one or more procedures on the workpieces. Other processing machines have a plurality of processing stations that perform a series of different procedures on individual workpieces or batches of workpieces. In a typical fabrication process, one or more layers of conductive materials are formed on the workpieces during deposition stages. The workpieces are then typically subject to etching and/or polishing procedures (i.e., planarization) to remove a portion of the deposited conductive layers for forming electrically isolated contacts and/or conductive lines.
Plating tools that plate metals or other materials on the workpieces are becoming an increasingly useful type of processing machine. Electroplating and electroless plating techniques can be used to deposit nickel, copper, solder, permalloy, gold, silver, platinum and other metals onto workpieces for forming blanket layers or patterned layers. A typical metal plating process involves depositing a seed layer onto the surface of the workpiece using chemical vapor deposition (CVD), physical vapor deposition (PVD), electroless plating processes, or other suitable methods. After forming the seed layer, a blanket layer or patterned layer of metal is plated onto the workpiece by applying an appropriate electrical potential between the seed layer and an electrode in the presence of an electroprocessing solution. The workpiece is then cleaned, etched and/or annealed in subsequent procedures before transferring the workpiece to another processing machine.
FIG. 1 illustrates an embodiment of a single-wafer processing station 1 that includes a container 2 for receiving a flow of electroplating solution from a fluid inlet 3 at a lower portion of the container 2. The processing station 1 can include an anode 4, a plate-type diffuser 6 having a plurality of apertures 7, and a workpiece holder 9 for carrying a workpiece 5. The workpiece holder 9 can include a contact assembly having a plurality of electrical contacts for providing electrical current to a seed layer on the surface of the workpiece 5. The seed layer acts as a cathode when it is biased with a negative potential relative to the anode 4. The electroplating fluid flows around the anode 4, through the apertures 7 in the diffuser 6, and against the plating surface of the workpiece 5. The electroplating solution is an electrolyte that conducts electrical current between the anode 4 and the cathodic seed layer on the surface of the workpiece 5. Therefore, ions in the electroplating solution plate onto the surface of the workpiece 5.
The plating machines used in fabricating microelectronic devices must meet many specific performance criteria. For example, many processes must be able to form small contacts in vias that are less than 0.5 μm wide, and are desirably less than 0.1 μm wide. The plated metal layers accordingly often need to fill vias or trenches that are on the order of 0.1 μm wide, and the layer of plated material should also be deposited to a desired, uniform thickness across the surface of the workpiece 5.
The structure of the contact assembly can significantly influence the uniformity of the plated metal layer because the plating rate across the surface of the microelectronic workpiece is influenced by the distribution of the electrical current (the “current density”) across the seed-layer. One factor that affects the current density is the distribution of the electrical contacts around the perimeter of the workpiece. In general, a large number of discrete electrical contacts should contact the seed-layer proximate to the perimeter of the workpiece to provide a uniform distribution of current around the perimeter of the workpiece. Another factor that affects the current density is the formation of oxides on the seed-layer. Oxides are generally resistive, and thus oxides reduce the efficacy of the electrical connection between the contacts and the seed-layer. Still other factors that can influence the current density are (a) galvanic etching between the contacts and the seed-layer, (b) “theiving” of material near the contacts caused by plating on the contacts during a plating cycle, (c) gas bubbles on the seed-layer, and (d) other aspects of electroplating that affect the quality of the connection between the contacts and the seed-layer or the fluid dynamics at the surface of the workpiece. The design of the contact assembly should address these factors to consistently provide a desired current density across the workpiece.
One type of contact assembly is a “dry-contact” assembly having a plurality of electrical contacts that are sealed from the electroplating solution. For example, U.S. Pat. No. 5,227,041 issued to Brogden et al. discloses a dry contact electroplating structure having a base member for immersion into an electroplating solution, a seal ring positioned adjacent to an aperture in the base member, a plurality of contacts arranged in a circle around the seal ring, and a lid that attaches to the base member. In operation, a workpiece is placed in the base member so that the front face of the workpiece engages the contacts and the seal ring. When the front face of the workpiece is immersed in the electroplating solution, the seal ring prevents the electroplating solution from contacting the contacts inside the base member. Other types of dry contact assemblies are disclosed in U.S. Pat. Nos. 6,139,712, and 6,309,524.
One manufacturing concern of dry-contact assemblies is that galvanic etching occurs between the contacts and the seed-layer when an electrolyte solution gets into the dry contact area. Galvanic etching removes the seed-layer at the interface of the contacts, which can cause a non-uniform current distribution around the perimeter of the workpiece. Therefore, even though dry-contact assemblies keep the contacts clean, they may produce non-uniform metal layers on the workpieces.
Another type of contact assembly is a “wet-contact” assembly having a plurality of electrical contacts that are exposed to the electroplating solution during a plating cycle. Because the contacts are exposed to the electroplating solution during a plating cycle, the metal in the electroplating solution also plates onto the contacts. The contacts, however, may plate at different rates such that some contacts can have a greater surface area of conductive material contacting the seed-layer. The in-situ plating of contacts can accordingly reduce the uniformity of the metal layer on the workpiece. Additionally, wet-contact assemblies must be periodically “de-plated” to remove the metal that plates onto the contacts during a plating cycle.